1. Field of the Invention
The present invention relates to a structure of piezoelectric type electric acoustic converters, such as a piezoelectric earphone, a piezoelectric sounding device, a piezoelectric speaker, and a piezoelectric buzzer, especially a diaphragm thereof.
2. Description of the Related Art
Conventionally, a piezoelectric type electric acoustic converter is widely used for a piezoelectric earphone, a piezoelectric buzzer, or other suitable piezoelectric device. A common structure of this piezoelectric type electric acoustic converter is that a metal plate of circular form is bonded on one side of a circular piezoelectric ceramic board to provide a unimorph type diaphragm, and a circumference of this diaphragm is supported in a circular case, and an opening of the case is closed with a cover. However, in the unimorph type diaphragm, there is a defect in that the displacement magnitude produced by the sound pressure is small, since the bending vibration is obtained by attaching the ceramic board, the outer diameter of which expands in response to the application of the voltage, to the metal plate which does not change dimensions.
In addition, a bimorph type diaphragm of the laminate structure including a plurality of piezoelectric ceramics layers is proposed in the unexamined Japanese patent publication No. 61-205100 gazette. This diaphragm is constructed by laminating a plurality of ceramic green sheets and a plurality of electrodes, and using the sintered compact body which is obtained by baking the sheets and electrodes at the same time. The electrodes are electrically connected via the through-holes formed at a position which does not constrain the vibration of a diaphragm. Compared with the unimorph type, the amount of larger displacement, i.e., larger sound pressure, can be obtained by arranging first and a second vibrating regions in the thickness direction in order so that they may vibrate in a reversed direction mutually.
However, in a case of the above-described bimorph type diaphragm, for example, if the bending vibration of the diaphragm including three ceramic layers is carried out, as shown in FIG. 17 of the above publication, the electrode of one main surface and one internal electrode should be mutually connected via a through hole. Another main surface electrode and another internal electrode need to be mutually connected via a through hole, and an alternating voltage needs to be applied therebetween. Therefore, the complicated interconnection between the main surface electrode and the internal electrode is necessary, and the cost thereof may become expensive.
Consequently, the applicant of the present invention eliminates the interconnection of the main-surface electrode and the internal electrode, and provides a piezoelectric type electric acoustic converter which defines a bimorph type diaphragm from a simple connection structure (in Japanese Patent Application No. 11-207198 which has not been published yet). This electric acoustic converter is characterized in that two or three piezoelectric ceramic layers are laminated to form a laminate body, main surface electrodes are formed on front and back main surfaces of this laminate body, internal electrodes are formed between respective ceramic layers, and all the ceramic layers are polarized in the thickness direction in the same direction. The bending vibration of the laminate can be performed by applying an alternating signal between the main-surface electrode and the internal electrode.
In the case of such a bimorph type diaphragm, there is a feature that larger sound pressure can be obtained compared with a unimorph type diaphragm. On the other hand, the shock resistance is low since there was no reinforcement by the metal plate, and when it is used for a portable terminal or other such uses, sufficient shatter resistance strength was not obtained.
In order to overcome the problems described above, preferred embodiments of the present invention provide a bimorph type diaphragm which obtains a large sound pressure while having a simple connection structure, and to provide a piezoelectric type electric acoustic converter which achieves a much greater improvement in shatter strength.
According to a first preferred embodiment of the present invention, a piezoelectric sound converter includes piezoelectric ceramic layers which are laminated to form a laminate, and main surface electrodes disposed on front and back main surfaces of the laminate, an internal electrode disposed between respective ceramic layers, the ceramic layers being polarized in the same direction, and the bending vibration of the laminate is performed by applying an alternating signal between the main surface electrode and the internal electrode. The front and back surfaces of the laminate are substantially completely covered with a resin layer.
If an alternating voltage is applied between the main surface electrode and the internal electrode in the laminate of this preferred embodiment of the present invention, the direction of an electric field acting in the ceramic layer of the front side and the back side will turn into a reverse direction in the thickness direction. On the one hand, the polarization direction of all ceramic layers have the same direction in the thickness direction. A piezoelectric ceramic used for the layers has the characteristic of shrinking in a direction of a flat surface if the direction of a polarization and the direction of an electric field are the same direction, and if the direction of a polarization and the direction of an electric field are reverse directions, it has the characteristic of extending in a direction of a flat surface. Therefore, when the alternating voltage is applied as mentioned above and the ceramic layer of the front side is expanded (shrunk), the ceramic layer of a back side will be shrunk (expanded) and a laminate will generate the bending vibration as a whole. Because this amount of displacement is larger compared with a unimorph type diaphragm, sound pressure is greatly increased.
The conventional laminate including ceramics is weak against an external shock applied thereto, while a sound pressure thereof is large. In preferred embodiments of the present invention, the laminate is reinforced by covering almost all of the front and back surfaces of a laminate with the resin layer, thereby greatly increasing the shatter resistance strength. Because this resin layer does not inhibit the bending vibration of the laminate, a sound pressure is not affected and a resonance frequency is not increased.
According to a second preferred embodiment of the present invention, the resin layer may be a stiffened coating layer provided after coating a paste-like resin in a film state. Alternatively, the resin layer may be a resin film attached to the laminate according to a third preferred embodiment of the present invention.
The resin material for forming a resin layer does not have the reinforcement effect of a laminate when it is a resin material with a low Young""s modulus, such as a silicone group and a urethane group. Also, the resistance to external shock cannot be expected sufficiently. With a resin material with high Young""s modulus, such as an epoxy group and an acrylic type, the shock resistance is greatly increased. As such materials, for example, polyimide resin, polyamide-imide resin, etc., are included.
According to a fourth preferred embodiment of the present invention, it is desirable to provide a laminate having a substantially rectangular shape. In a substantially rectangular laminate, processes, such as forming an electrode, laminating ceramic layers, a press attachment, baking, and a formation of a resin layer, can be performed in a stage of a mother board, so that material waste is minimized while mass production efficiency is greatly improved. Furthermore, when a substantially rectangular diaphragm is provided, sound conversion efficiency is greatly improved compared with a circular diaphragm, and there is an advantage that a low frequency sound can be generated.
It is sufficient to make the main surface electrodes of the front and back surfaces conduct mutually via the first side electrode formed on the side of the laminate as in a fifth preferred embodiment of the present invention, and to make the internal electrode conduct with the second side electrode formed on the side of a different position from the first side electrode. In this case, the electric connection with the exterior becomes simple by pulling out the main-surface electrode and the internal electrode via the side electrode.
It may be preferable to form the first and second side electrodes so that they may turn to the front and back surfaces of a resin layer according to a sixth preferred embodiment of the present invention. For example, when connecting the electric acoustic converter of the present invention electrically with the exterior, using electroconductive glue, or other material, the connection thereof becomes simple and the formation of the electrodes becomes simple.
According to a seventh preferred embodiment of the present invention, the second side electrode may turn to the front and back surfaces of the laminate. The notch part where a part of main surface electrodes of the front and back surfaces exposes, and the notch part where a part of the second side electrode turning to the front and back surfaces of the laminate exposes, may be formed with a resin layer.
In this case, it is not necessary to form an electrode on the surface of a resin layer as in the sixth preferred embodiment of the present invention, only an electrode is required to be formed on the laminate, so not only the electric connection with the exterior becomes simple, but also operation of an electrode formation becomes simple.
Other features, elements, characteristics and advantages of the present invention will become apparent from the detailed description of preferred embodiments thereof with reference to the attached drawings.